Can't update / remove keyframe values from an imported Collada file
The problem seemed to be that the imported collada file had multiple sets of clones of the same keyframes - 5 in my case. I'm not sure if this is a problem with the current collada import script or just with certain files, but I managed to fix the file by deleting all but the last set of cloned keyframes for each bone via the dopesheet. Once this was done I could successfully change the Quaternion channels and keyframe them as per normal. The imported animation is preserved and now editable.